1. Field of the Invention
This invention relates to anti-staining gear oils for use in lubricating aluminum rolling mill gear and beating system and to reducing stray mist formation by such oils in the course of their use.
2. Description of the Prior Art
In aluminum rolling applications, gear oils used in systems where there is the potential of contaminating the rolling lubricant (which is applied to the surface of the aluminum being rolled), often require aluminum anti-staining (low staining) as a property. Furthermore, in some applications, for example the manufacture of aluminum foil for packaging or use in home kitchens, the gear oil has to satisfy Food and Drug Administration (FDA) food grade requirements, which impose restrictions on base stocks and additives utilized in the gear lubricant.
Polybutenes (PB's, copolymers made from isobutylene and butene monomers) have been commercially used as base stocks for aluminum anti-staining gear oils. These are mixtures of low molecular polymer grades (typically less than 2,500 number average), blended to meet the viscosity target of the lubricant. The PB base stocks would meet FDA requirements under 21 CFR 178.3570. The performance additives utilized in the formulated lubricant are low staining, and FDA compliant or non-compliant, depending on the requirements of the particular end use application.
Polyethers (FDA-compliant) and alkyl benzenes (non FDA-compliant) have also been used as base stocks for aluminum anti-stain lubricants.
Gear box and beating systems are often lubricated from a sump full of oil or a centralized oil circulation system. Stray mist formation is not a concern in these situations. Mist lubrication systems, i.e., systems where gear oils are converted into an aerosol mix in air and pneumatically delivered to the lubrication point in the form of an oil mist, have gained increased popularity since the 1960's. At the point of lubrication, devices called "reclaimers" coalesce the oil into larger droplets, and deliver it to the equipment being lubricated. It is desirable for the mist lubricant to form a low level of "stray mist" to protect the environment at the vicinity of the lubricated equipment from fogging.
Commercial mist lubricants range in ISO viscosity grades 68 to 680, more commonly ISO 100 to 460.
U.S. Pat. No. 3,510,425 teaches means to reduce stray mist in applications where mineral oil-based gear oils are utilized in a mist-type lubrication system. U.S. Pat. No. 3,5 10,425 teaches that using 0.05% to 3.5% of an oil soluble polyester of between about 80,000 to 150,000 number average molecular weight, made by esterification of C.sub.12 -C.sub.20 alkyl monohydric alcohols and a mono-unsaturated mono carboxylic acids such as acrylic or methacrylic acids, is very effective in reducing generation of undesirable stray mist in mineral oil based mist lubricating oils. U.S. Pat. No. 3,510,425 states that, among many polymers tested unsuccessfully, polyisobutylenes (PIB's) of 130,000 number average molecular weight at 0.5 to 1.5% treat rates were not effective in reducing stray misting by mineral oil-based lubricants.
While methacrylate-type polymers used as mist control agents in mineral oil-based gear oils also function in PB based low stray misting oils, it is desirable to achieve yet higher stray mist suppression. Furthermore, use of polymethacrylates is not allowed in gear oils which need to meet Food Grade Administration (FDA) food grade requirements.
Aluminum rolling systems with mist lubricated bearings or gears therefore currently either use mineral oil-based mist oils containing mist reducing additives and accept a stain debit (and non-FDA debit, where applicable), or use PB-based gear oils which are non-staining but are accompanied with a debit in stray mist.